Photolithography processes are widely used in the fabrication of integrated circuits. In a typical photolithography process, a thin layer of a photoactive material called “photoresist” is deposited onto the surface of a semiconductor substrate, such as a silicon wafer. The photoresist can be composed of any of a number of different materials whose chemical structure changes when exposed to a certain type of radiation. This change in chemical structure can cause the photoresist to become more soluble, in the case of a positive photoresist, or less soluble, in the case of a negative photoresist, in a chemical solution referred to as a developer.
A photolithography mask in the shape of a desired circuit pattern is used as a template to transfer the circuit pattern to the surface of the semiconductor substrate. A typical transmission mask has a pattern of clear and opaque areas, repeated over its surface, that is used to fabricate a layer of a circuit. The mask, when positioned between an appropriate radiation source and the photoresist-coated semiconductor substrate, casts a shadow onto the photoresist and thus controls which areas of the photoresist are exposed to the radiation. On other types of masks reflect light, a reflective pattern reflects light toward the semiconductor substrate.
After exposure, the photoresist is removed from either the exposed or the unexposed areas by washing with an appropriate developer. This leaves a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent process step, such as etching, deposition, or diffusion. After the subsequent process step is completed, the remaining photoresist is removed. This photolithography process ultimately allows the actual circuitry to be integrated into a semiconductor chip.
Obviously, the mask is a key element in this process since it is the mask that determines the circuit pattern that is transferred to the semiconductor substrate. A typical mask comprises a patterned layer of an opaque absorber material, such as a metallic film of chromium or tungsten, on a substrate of a transparent material, such as quartz. Features on a mask can be as small as a few millionths of an inch. When the pattern is formed on the mask, typically by using computer controlled laser or electron beam systems to expose the desired mask pattern in a photoresist material, it is not unusual for the mask to have defects. There are essentially two defect types, opaque and clear. Clear defects are areas where absorber is missing from areas that should be opaque; opaque defects are areas having absorber material deposited in areas that should be clear. Since any defect in the mask will ultimately be transferred to any semiconductor chip manufactured using that mask, these defects must be repaired before the mask can be used.
Traditionally, focused ion beam systems (FIB) have been used to repair defects in photolithography masks. A finely focused beam of gallium ions from a liquid metal ion source is scanned across the mask surface to form an image of surface. The intensity at each point of the image is determined by the current of secondary electrons ejected by the ion beam at the corresponding point on the substrate. The defect is identified on the image, and the ion beam is then scanned over the defect area in order to remove the excess absorber material from a mask surface or to deposit missing absorber material.
When used to remove material, the heavy gallium ions in the focused ion beam physically eject atoms or molecules from mask surface by sputtering, that is, by a transfer of momentum from the incoming ions to the atoms at the surface. The momentum transfer mechanism is considered to function through a series of collisions with nuclei in the substrate lattice, the process being referred to as a “collision cascade.”
When a FIB is used to deposit material to repair a clear defect, a gas is directed toward the defect area, and material is deposited by using an ion beam to decompose gas molecules absorbed on the substrate surface. A process for depositing a metal material using a FIB is described, for example, in U.S. Pat. No. 5,104,684 to Tao entitled “Ion Beam Induced Deposition of Metals.”
There are several problems with the use of gallium ion FIB systems to repair masks, particularly when used to repair opaque defects. First, gallium ions become implanted into the substrate surrounding the defect area. This phenomenon, commonly referred to as “staining,” causes the stained substrate to lose some of its transparency. This loss of transparency, in turn, introduces defects in the mask image that is transferred to the semiconductor substrate. The loss of transparency is particularly severe for the very short exposing light wavelengths used in modern photolithography processes, with the loss of transparency typically being between three and ten percent.
Second, the sputtering process of the focused ion beam is relatively unselective. While an opaque defect is being removed by the ion beam, substrate material at the edge of the defect is also attacked, and the result is a trench of damaged substrate material around the defect. This type of substrate damage is known as “riverbedding” because the etched edges resemble riverbeds when viewed with an electron microscope. Riverbedding results in an altered intensity and phase for the light traversing the quartz surrounding the defect.
Third, the sputtering of material by the ion beam leads to ejection of material in all directions, and some of this ejected material comes to rest on adjacent surfaces. This effect, known as redeposition, limits the precision of the microstructure fabrication.
Lastly, because the mask substrate is typically made of an insulating material, a positive electrical charge tends to accumulate on isolated defects when they are bombarded by the positive gallium ions. Each positively charged gallium ion not only brings a positive charge to the area, each massive ion also ejects multiple electrons from the surface. As this positive charge accumulates, it will reduce the emission of secondary electrons by which an image of the defect is attained. Ion beam systems used for mask repair typically include a charge neutralizer, such as an electron flood gun as described in U.S. Pat. No. 4,639,301 to Doherty, et al. for “Focused Ion Beam Processing.” It can be difficult to adjust the flood gun to just neutralize the surface charge, especially as the surface composition under bombardment is changing as the absorber material is removed.
Sputtering by an FIB system can be further enhanced, and some of the previously described problems can be minimized, by using an etching gas that adsorbs onto the surface and reacts to form volatile compounds with the surface atoms under impact of the ion beam. The surface atoms are then more readily removed and less likely to redeposit. The gas atoms react with the surface molecules when energy is provided by the incoming ions. The incoming ions do not significantly react directly with the adsorbed gas molecules. The ions typically react in a series of collisions with atoms in the substrate, the collisions providing energy back through the lattice to knock atoms from the surface and instigate chemical reactions with the adsorbed gas molecules. Some gases cause the ion beam to preferentially etch one material over another. Although the use of a gas can reduce the enumerated problems associated with gallium-based FIB systems, the problems still remain.
Some materials are known to be etched by an etchant chemical in the presence of an electron beam. Electrons cannot sputter material because the momentum of an electron in a typical electron beam is not sufficient to remove molecules from a surface by momentum transfer. The amount of momentum that is transferred during a collision between an impinging particle and a substrate particle depends not only upon the momentum of the impinging particle, but also upon the relative masses of the two particles. Maximum momentum is transferred when the two particles have the same mass. When there is a mismatch between the mass of the impinging particle and that of the substrate particle, less of the momentum of the impinging particle is transferred to the substrate particle. A gallium ion used in focused ion beam milling has a mass of over 130,000 times that of an electron. In a typical focused ion beam system, the gallium ions are accelerated through a voltage of 25-50 kV, whereas the electrons in a transmission electron microscope are typically accelerated through a voltage of 100 kV. The momentum transfer of a typical 30 kV gallium ion impinging on a copper substrate in a FIB system is therefore greater than 20,000 times that of a 100 kV electron in an electron microscope.
Chemically induced etching using an electron beam therefore occurs through a different mechanism than the mechanism of ion beam sputtering. An electron beam will not etch in the absence of a chemical etchant, whereas an ion beam will always sputter material, even though sputtering may be enhanced or attenuated by a gas. Thus, an electron beam cannot be used to etch a particular material unless a chemical is found that will etch the material in the presence of the electron beam, but will not significantly etch the material in the absence of electron beam.
Several etchants are known for use with electron beams. Matsui et al. in “Electron Beam Induced Selective Etching and Deposition Technology,” Journal of Vacuum Science and Technology B, Vol. 7, No. 5 (1989) describes electron beam induced etching of silicon, gallium arsenide, and polymethylmethacrylate using xenon difluoride, chlorine, and ClF3. Matsui et al. also describe the deposition of tungsten using tungsten hexafluoride in the presence of an electron beam. Winkler et al. in “E-Beam Probe Station With Integrated Tool For Electron Beam Induced Etching,” Microelectronic Engineering 31, pp. 141-147 (1996) describes electron beam induced etching of insulation layers, such as SiO2, Si3N4, and polyimide on integrated circuits. U.S. Pat. No. 6,753,538 to Musil et al. for “Electron Beam Processing” teaches the use of an electron beam to etch a film containing tantalum and tungsten. U.S. Pat. App. Pub No. 2003/0000921 of Liang et al. for “Mask Repair with Electron Beam-Induced Chemical Etching” describes etching tantalum nitride using XeF2, CF4, or F2 and etching chrome compounds using Cl2 and O2. U.S. Pat. No. 5,055,696 to Hirachi et al. for “Multilayered Device Micro Etching Method and System” describes locally etching material using a reactive gas or a focused ion beam using a variety of etchant gases. Hirachi et al. mentions that an electron beam or a laser could be used in place of the ion beam, but it fails to describe which etchants will etch in absence of ion beam sputtering and his detailed description of end-pointing is indicative of ion beam, not electron beam, processing.
The etchants described above do not etch certain material, such as chromium materials, used on photolithograph masks with the desired speed and selectivity.